For many years laminates have been used as a surfacing material in residential and commercial structures. Typical applications of said laminates are surfacing for walls, partitions, table tops, counter tops, furniture, doors and the like. Such laminates generally are produced from a plurality of resin impregnated core sheets usually composed of kraft paper which has been impregnated with a thermosetting resin and, more particularly, with a thermosetting water-soluble or water-insoluble phenolic resin. When the kraft paper has been impregnated with the thermosetting resin, the sheets are dried and cut to the appropriate size. Thereupon, a plurality of these resin impregnated sheets are stacked in a superimposed relationship. The number of plies or sheets in the stack depends on the ultimate intended use of the laminate. For most purposes, the number of plies of these core sheets will total about three to nine but can total as many as 15-20.
There is then placed on the stack of core sheets a decorative sheet which is generally a sheet of paper bearing a printed design or of a solid color and impregnated with a noble thermosetting resin which is not subject to significant darkening upon the application of heat. Suitable resins for the decorative sheets are the aminotriazine resins and more particularly the melamine-formaldehyde resins, the benzoguanamine-formaldehyde resins and the unsaturated polyester resins and the like. It is generally desirable when making decorative laminates to make use of a protective overlay sheet which is placed atop and is similar to the decorative sheet but is generally devoid of design and in the final laminate is transparent. The stack of impregnated sheets is inserted into a laminating press and is heat and pressure consolidated to a unitary structure. During the consolidation step, the thermosetting resins are converted to the thermoset state thereby providing an extremely hard, attractive and permanent surfacing material. For obvious economic reasons, it is common practice to consolidate a plurality of these individual laminating assemblies into one large assembly, or press pack, said stacks being separated from one another by a release sheet, and then to laminate this pack by heat and pressure application.
The press pack is then pressed or molded. This is accomplished by placing the pack between the platens of a hydraulic press. The press usually has multiple openings so that many packs may be pressed at once. Such an approach produces a very smooth surface laminate with a glossy finish. Other techniques are used to produce laminates with surface finishes ranging from a satin or brushed textured look to a deeply embossed effect.
While these laminates have found commercial success, especially in the area of decorative applications, they suffer from one defect which has restricted their use in such areas as computer assembly work stations, floors, walls, ceilings; electronic furniture such as cabinetry; commercial floors of hotels etc; hospital and laboratory rooms wherein electrostatically sensitive films, solid state devices and hazardous vapors and the like are handled, i.e. areas where static build-up on surfaces causes problems. This defect is the laminates' inability to control electrostatic currents.
U.S. Pat. No. 4,301,040 provides a static-free laminate, wherein the back surface of a conventional decorative laminate is coated with a polymeric, film-forming, particulate binder material having uniformly dispersed therein a static-reducing amount of electrically conductive particle material such as carbon black.
Although the laminates of the above-mentioned patent partially solve the problem of static build-up, they require conductive particle bonding with adhesives which causes particle removal when contact with the surface occurs. Additionally, delamination of the conductive particle surface oftimes occurs when the laminate is contacted with materials such as water. Furthermore, working of the laminates such as by sawing, drilling etc. tends to further remove said particles of electrically conductive material. Another problem with the laminates of said reference is the difficulty with which uniform amounts of the particulate conductive material can be applied to the adhesive coated surface.